Effect of Cr on structural and magnetic properties of cobalt ferrite
R.K. Panda and D. Behera*
National Institute of Technology, Rourkela, Odisha-769008.
Conclusion:
Cr substituted cobalt ferrite nanoparticles was prepared by the auto combustion method. The phase was confirmed by the XRD analysis and the lattice constant decreased with increasing Cr addition. The particle size in nano order was estimated from the surface morphology study of FESEM image. Particle size, saturation magnetization and coercivity were decreased with increasing percentage of Cr to parent material .Fig.2.Surface morphology of a)CoFe2O4 b)CoFe1.9Cr0.1 O4 c) CoFe1.8Cr0.2O4
Fig.4.Field dependent magnetic properties of CoFe1-xCrxO4 at 300 K.
Introduction
Magnetic systems with spinel structure have generated interest for its role in wide range of applications due to their large spin polarization and high magnetic critical temperatures, typically well above the room temperature.
Ferrimagnetic and electrical properties of the spinel ferrites are strongly influenced by the distribution of cations along with Fe3+-Fe2+ between the tetrahedral and octahedral sites. Particularly, cobalt ferrite, inverse spinel at bulk and partial inversion in nano order. The inter play of cations in site occupation tune the electric and magnetic properties of the cobalt ferrite. In the present study we tried to tune the magnetic properties of the cobalt ferrite by substituting the Cr3+ in place of Fe.
Fig.1.Crystal structure and spin directions at tetrahedral and octahedral sites.
[Co
1-xFe
x]
A[Co
xFe
2-x]
BO
4A and B represents tetragonal and octahedral sites.
If x = 0 Normal spinal x =1 Inverse spinel
0< x <1 Partial inversion The net magnetization is M
A~M
BMagnetic Properties
Surface morphology& phase analysis
Fig.5. Variation of magnetic properties with Cr.
Fig.3. XRD pattern of CoFe1-xCrx O4
Effect of Cr on structural and magnetic properties cobalt ferrite
D. Behera
Department of Physics and Astronomy NIT Rourklea
Plan of Presentation
Introduction on Ferrites
About cobalt ferrite
Materials and methods
Characterization
Magnetic properties
Conclusion
Introduction
Ferrites belongs to ferrimagnetic
group which exhibit net magnetization at room temperature.
Magnetite (Fe3O4) is the naturally occurring ferrimagnetic material.
Para
Dia Ferro Antiferro
Ferri magnetic
Magnetic materials
Ferri magnetic material
Classification of Ferrites
M1113Fe5O12 M11Fe12O19
Ferrites
Spinel
Hexagonal Garnets
M11Fe2O4
Depending on the site occupancy, spinels are classified into : -normal -inverse
Fig. Unit cell of normal spinel
Some times inversability is not complete, is known as partial inversability
In partial inversability site occupation [M 1-x Fex]A [Mx Fe2-x]B O4where
A and B represents tetragonal and octahedral sites and x represents the inversability.
If x=0 Normal spinal x=1 Inverse spinel 0<x<1 partial inversion
The net magnetization is MA~MB
Materials and methods
Cr substituted cobalt ferrite nanoparticles was prepared by the auto combustion method
CoFe1-xCrxO4 with varying x (x = 0, 0.05, 0.1, 0.15, 0.2)
CFO nanoparticles were prepared by the auto combustion method. Analytical grades cobalt nitrate Co(NO3)26H2O, ferric nitrate Fe(NO3)39H2O were taken as oxidants while glycine C2H5NO2 was employed as fuel to drive the combustion reaction.
Phase analysis
XRD of CoFe1-xCrxO4 with varying x
Lattice constant decreasing because lower size Cr3+ (0.615 A) substituting the heavier atom Fe3+ (0.645 A).
Particle size decreases with increasing addition of Cr3+
Particle size decrease may be due to strain induced by the decrease in lattice constant Surface morphology of a)CoFe2O4 b)CoFe1.9Cr0.1O4 c) CoFe1.8Cr0.2O4
Magnetic properties
Field dependent magnetic properties of CoFe1-xCrxO4 with varying x at 300 K.
Compositional dependence of magnetic properties
The tetrahedral and octahedral sub- lattices of ferrite may be subdivided in such a way that the vector resultants of the magnetic moments of the sub-lattices are aligned in such a direction that will influence the effective magnetization. Thus the decrease in magnetization could be explained on the basis of non- collinear spin arrangement that arose due to the substitution of Cr ions.
Ms and Hc for CoFe1-xCrxO4 at 300 K.
In CFO system, the structure transforms from partial inverse to inverse spinel as particle size changes from nano to bulk range.
Therefore, the magnetic properties must change accordingly with the growth of particle size. Cobalt occupies the octahedral site in inverse spinel whereas it occupies both the sites in partial inverse spinel form.
As Chromium substitution brings decrease in particle size therefore, there may be the migration of cations from one site another site which leads to decrease in saturation magnetization.
The coercivity of fine particles has striking properties on their size.
As the grain size decreases, the coercivity increases, goes through a maximum then decreases and become zero for a very small size known as superparamagnetism.
The change in coercivity is due to the change of the multidomain nature to single domain. In the multidomainregion, the size dependence of coercivity is expressed as
�� = + �
Where D is diameter of the particle and a, b are constants.
Mössbauer spectroscopy probes tiny changes in the energy levels of an atomic nucleus in response to its environment.
Three types of nuclear interactions may be observed:
Isomeric shift, also known as a chemical shift Quadrupole splitting; and
Magnetic or hyperfine splitting, also known as the Zeeman effect.
Due to the high energy and extremely narrow line widths of gamma rays, Mössbauer spectroscopy is a very sensitive technique in terms of energy (and hence frequency) resolution, capable of detecting change in just a few parts per 1011
Mossbauer Studies
Mossbauer Studies for the Cr substituted CFO
Separate contributions due to the A tetrahedral and B octahedral sites can be identified in the Mossbauer spectra.
In this study, Mossbauer spectroscopy was used to investigate the local environments of the Fe atoms in these materials. A series of five
powder samples with
compositions CoCrx Fe2−xO4
It would appear that Cr substitutes into the B sites and has the effect of displacing Co ions onto the A sites.
A typical range of velocities for 57Fe, for example, may be ±11 mm/s (1 mm/s
= 48.075 neV)
Conclusions
Cr substituted cobalt ferrite nanoparticles prepared by the auto combustion method.
The phase was confirmed by the XRD analysis and the lattice constant is decreased with increasing Cr addition.
The particle size of Nano order was estimated from the surface morphology study of FESEM image.
Particle size was decreased with increasing percentage of Cr and decrease in saturation magnetization as well as coercivity.
Further studies on Mossbauer to show several aspects on magnetic properties
Thank you for attention
